Patent GB-1023053-A (GB)
Title: Improvements in or relating to receivers for an aircraft landing system
Inventor: BENJAMIN JOHN
IPC: G01S1/02
Filed: 1963-09-13
Abstract: 1,023,053. Radio navigation. MINISTER OF AVIATION. Aug. 28, 1964 [Sept. 13, 1963; Sept. 23, 1963], Nos. 36164/63 and 37237/63. Heading H4D. The reliability of I.L.S. signals, comprising two double sideband guidance signals, the relative depths of modulation of which are intended to indicate the position of the receiver relative to a preferred approach path, is tested by means obtaining separate measurements of divergence from the preferred path from the upper sideband components and from the lower sideband components of the two double sideband signals and for indicating a discrepancy between the two measurements. The invention is based on the assumption that interference will effect the high and low sidebands of the signals differently. In the receiver of Fig. 1, the two received signals are heterodyned down in a double superheterodyne assembly to produce two I.F. signals comprising carriers of 2 Mc/s. and sidebands of 2 Mc/s.  90 c/s. and 2 Mc(s.  150 c/s. The two signals are detected by means 9, to produce two signals at frequencies of 90 c/s and 150 c/s. These are mixed in mixer 19 to produce signals at frequencies of 60 c/s. and 240 c/s. which are frequency divided by means 20 and 21 to produce signals at frequencies of 30 and 120 c/s. These signals then pass through phase shifters 23 and 22 to compensate for any phase shifter in the system. The 120 c/s. signal is used to modulate a 2 Mc/s. signal, produced by filtering out the carrier from the I.F. signals, in single side band modulators 15 and 16. Lower and upper sideband signals of 1,999,880 c/s. and 2,000,120 c/s. respectively are produced by the two moduators and are mixed in mixers 12 and 13 with the I.F. signals. The mixers are provided with output filters tuned to 30 c/s. The two signals at 30 c/s. produced by mixer 12 are thus produced by mixing the signal at 1,999,880 c/s. with the lower side band signals at frequencies of 2 Mc/s.-90 c/s. and 2 Mc/s.-450 c/s. The two 30 c/s. signals are in antiphase, such that the amplitude of the mixer output represents the required lower sideband difference in depth of modulation. Similarly the 30 c/s. output of mixer 13 is produced by the mixing of the signal at 2,000,120 c/s. with the upper sideband signals and has an amplitude representing the upper sideband difference in depth of modulation. The two mixer outputs are detected in phase-sensitive detectors 17 and 18, using the 30 c/s. signal from phase shifter 23 as the reference signals. The two outputs of the detectors, which should be equal, are fed to a difference indicator 24, to display any difference, and show the presence of interference. Switch 25 enables either of the detector outputs to be fed to a standard I.L.S. indicator 26. In the receiver of Fig. 2 (not shown), the 2 Mc/s. carrier is modulated in the single sideband modulators 15 and 16 with a 240 c/s. signal obtained from mixer 19, to produce two signals at frequencies of 2,000,240 c/s. and 1,999,760 c/s. These signals are mixed in mixers 12 and 13 with the I.F. signals. The two mixers are provided with output filters, passing signals at 90 and 150 c/s. only. The output of mixer 12 therefore results from mixing the signal at 2,000,240 c/s. with the two upper side-band signals at frequencies of 2 Mc/s. + 90 c/s. and 2 Mc/s. + 150 c/s. The two components of the output are separated by filters 27 and 28, and are then detected by diodes 29 and 30, to produce, at the junction of resistors 31 and 32, a signal representing the upper side-band difference in depth of modulation. Similarly the two side-band signals produce the output from mixer 13 and produce a signal of the junction of resistors 37 representing the lower side-band difference in depth of modulation. The two difference signals are then treated as in the receiver of Fig. 1. In the receiver of Fig. 3 (not shown), the carrier is changed in frequency to 2,220,000 c/s., by means of a mixer 39 and a local 220 kc/s. oscillator 40. The altered carrier is then mixed with the I.F. signal in mixer 12 to produce four signals which are separated out by filters 41-44 tuned as indicated. Filters 41 and 42, deal with the two lower side-band signals, and filters 43 and 44 deal with the two upper side-band signals. The two difference in depth of modulation signals are produced and dealt with as before, excepting that both are used simultaneously to energize the I.L.S. display. The receiver of Fig. 4 (not shown), is similar to that of Fig. 3 (not shown), with the addition of a third heterodyne stage, reducing the carrier of the I.F. signal to 220 kc/s., a bandstop filter, removing the carrier from the I.F. signal, so that only the sideband signals remain for separation in the four filters, and a two-stage frequency control system which allows for changes in the carrier frequency due to doppler shifts &c. The first stage of the system comprises an L/C discriminator, determining the frequency of the final I.F. signal, and feeding in turn, an integrating amplifier, and a reactance modulator. The modulator then controls the frequency of the second oscillator 7. The second stage of the system uses a crystal discriminator and controls the frequency of the third heterodyne oscillator. Where the receiver is used in a ground monitor system a frequency drift alarm system may be added in parallel with the four filters, comprising a set of filters having pass frequencies just off 90 and 150 c/s., and feeding a set of alarms, Fig. 5 (not shown). The receivers may be used in a localizer system on a glide path system.